The present invention deals with a temperature control system for controlling equipment used to condition a building environment. More particularly, the present invention is a technique for controlling heating/cooling equipment based on a temperature error determined from a thermostat on/off signal.
Residential heating and cooling systems typically include a thermostat which is used to sense the temperature in a conditioned environment. The thermostat typically has a temperature set point input which allows an operator to specify a desired temperature. Based on the difference between the desired temperature, set by the operator, and the temperature sensed by the thermostat, the thermostat provides a control signal to heating/cooling equipment (a heating/cooling plant). The heating/cooling plant, in turn, operates to control the temperature in the conditioned environment based on direct control signal provided by the thermostat.
Residential heating and cooling plants are often controlled by electromechanical (E/M) thermostats. Since people in the conditioned environment not only feel air temperature but also feel heat radiated from the walls of the conditioned environment, the E/M thermostat "senses" a combination of the air and wall temperatures. This "composite sensed temperature" is compared to a desired temperature set point which is provided to the E/M thermostat. Based on the difference between the composite sensed temperature and the desired set point, the E/M thermostat provides an on/off signal to control the heating/cooling plant. E/M thermostats are cost effective and generally provide acceptable thermal comfort.
However, an E/M thermostat is a proportional controller. That is, the on/off signal generated by the E/M thermostat is proportional to the difference between the composite sensed temperature and the desired set point (i.e., it is proportional to the temperature error). Thus, an E/M thermostat requires some non-zero temperature error in order to take corrective action.
The temperature error is also called droop, and the droop increases with the thermal load of the environment being controlled. Hence, where the environment being controlled has a high thermal load, the temperature droop can be large enough to cause the controlled temperature in the conditioned environment to deviate from the desired set point by as much as 3.degree. Fahrenheit (F). In addition, where a multi-stage heating or cooling system is controlled with an E/M thermostat, the conditioned environment can experience as much as 3.degree. F. of droop per stage.
Another problem with E/M thermostats is that they typically use anticipator heaters, the current setting of which is typically set by moving a lever. Nominal cycling performance is only provided when the anticipator heater current setting matches the actual electric current flowing through the thermostat. If the anticipator setting and the current are "mismatched", undesirable on off cycling performance will result. Even a small difference between actual current and anticipator setting can cause a large change in thermostat on off cycling rate. Fixed-value resistor anticipators typically found in thermostats used to control residential cooling plants produce different cycling rates based on the voltage applied to the thermostat. Since voltages in different systems may range from 20 to 30 VAC, the resulting cycling rates will vary widely. It would therefore be desirable to automatically "correct" the thermostat cycling rate so that the heating or cooling plant is always cycled at the proper rate (e.g., 6 cycles per hour (cph) for gas warm air furnaces, 3 cph for air conditioners, etc.) regardless of the installation conditions. Thermostat cycling rates are normally specified at the 50 percent load (equal on- and off-times) condition.
With the advent of electronic, microprocessor-based thermostats, more sophisticated control algorithms have been employed to control heating/cooling plants. A proportional-plus-integral (PI) algorithm can be implemented in these thermostats to effectively eliminate the temperature droop. Although electronic thermostats offer droopless control, as well as other features such as seven-day programming of temperature set points, they are significantly more expensive than conventional E/M thermostats.
Therefore, it would be desirable to obtain PI (i.e., droopless) temperature control from an E/M thermostat at little or no additional cost. This would combine the improved temperature control of the microelectronic thermostat with the low cost advantage of the E/M thermostat.
Residential heat pumps with auxiliary heaters require two-stage thermostats. Two-stage microelectronic thermostats are available that provide droopless control, but these thermostats cost more than their E/M counterparts. Cheaper multi-stage E/M thermostats have the disadvantage of excessive droop as mentioned above. Therefore, it would be advantageous to obtain droopless multi-stage control from a single-stage E/M thermostat with little or no extra cost.
The problem of the "mismatched" anticipator is solved by the microelectronic thermostat, since the desired cycling rate can be produced in software regardless of the electric current or voltage applied to the thermostat. However, one of several cycling rates can still be selected by the homeowner or installer and thus it is still possible that the chosen thermostat cycling rate is not appropriate for the particular heating or cooling plant being controlled. Thus, it is also advantageous that the particular cycle rate applied to a heating or cooling appliance be guaranteed, regardless of the cycle rate setting on a microelectronic thermostat.
Several types of advanced heating/cooling systems are equipped with microprocessor-based electronic control units. Examples of such systems include high-efficiency furnaces and heat pumps, two-speed and variable-speed heat pumps, and zoning systems. These microprocessor-based electronic control units are capable of containing and executing various software programs.